IB Chemistry 3.4 Electron-pair sharing reactions Questions

IB Chemistry syllabusChemistry SL/HLFirst assessment 2025

Sharpen electron-pair reaction skills by classifying species, spotting heterolytic fission, and predicting products in substitution, addition, and Lewis acid-base questions.

Exam points

classify species in context: CN- nucleophile/Lewis base, CH3Br electrophile, Al in AlCl3 Lewis acid
identify heterolytic bond fission and distinguish it from homolytic bond breaking in mechanisms
predict products of pair-reaction steps, e.g. CH3CN from KCN/ethanol or 1,2-dibromoethane from Br2

[Maximum number: 4]

Nitrous acid, $\mathrm{HNO}_{2}$ , is a weak acid which can be used to make acidic buffers.

Chloroethane is then converted into ethanol using aqueous potassium hydroxide, KOH(aq).

Explain, using curly arrows, the mechanism of this reaction.

[Maximum number: 1]

Which species is the electrophile?

\mathrm{CH}_{3} \mathrm{Br}+\mathrm{OH}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{OH}+\mathrm{Br}^{-}

$\mathrm{OH}^{-}$

$\mathrm{Br}^{-}$ c. $\mathrm{CH}_{3} \mathrm{OH}$ D. $\mathrm{CH}_{3} \mathrm{Br}$

[Maximum number: 4]

Bromoethane can react with aqueous hydroxide ions to produce ethanol.

\mathrm{H}_{3} \mathrm{CCH}_{2} \mathrm{Br}(\mathrm{l})+\mathrm{OH}^{-}(\mathrm{aq}) \rightarrow \mathrm{H}_{3} \mathrm{CCH}_{2} \mathrm{OH}(\mathrm{aq})+\mathrm{Br}^{-}(\mathrm{aq})

(a)

State the type of reaction.

[ 1 ]

(b)

Sketch the mechanism of the reaction, showing structural formulas and using curly arrows to represent the movement of electron pairs.

[ 3 ]

[Maximum number: 1]

Which statements are correct for the complex ion $\left[\mathrm{FeCl}_{4}\right]^{2-}$ ?

I. Chloride ions are behaving as ligands.
II. The oxidation state of iron is +3 .
III. Iron ion forms coordination bonds with chloride ions.

I and II only

I and III only

II and III only

I, II and III

[Maximum number: 1]

What is the main product of the reaction between but-1-ene, $\mathrm{CH}_{2} \mathrm{CHCH}_{2} \mathrm{CH}_{3}$ , and hydrogen bromide, HBr ?

$\mathrm{CH}_{3} \mathrm{CHBrCH}_{2} \mathrm{CH}_{3}$

$\mathrm{CH}_{2} \mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$

$\mathrm{CH}_{2} \mathrm{BrCHBrCH}_{2} \mathrm{CH}_{3}$

$\mathrm{CH}_{2} \mathrm{BrCH}\left(\mathrm{CH}_{3}\right)_{2}$

[Maximum number: 1]

Which statement is correct about an $\mathrm{S}_{\mathrm{N}} 2$ mechanism?

Two molecules are involved in the rate-determining step of the mechanism.

Two steps are involved in the mechanism.

A nucleophile is substituted by an electrophile.

The mechanism involves formation of an intermediate carbocation.

[Maximum number: 8]

Bromoethane, $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br}$ , is produced by reacting ethene, $\mathrm{C}_{2} \mathrm{H}_{4}$ , with hydrogen bromide, HBr .

\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{~g})+\mathrm{HBr}(\mathrm{~g}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br}(\mathrm{l})

(a)

State the type of reaction and the role of HBr .

[ 1 ]

(b)

Outline why ethene is susceptible to attack by molecules such as HBr .

[ 1 ]

(c)

Bromoethane can react with aqueous hydroxide ions to produce ethanol.

\mathrm{H}_{3} \mathrm{CCH}_{2} \mathrm{Br}(\mathrm{l})+\mathrm{OH}^{-}(\mathrm{aq}) \rightarrow \mathrm{H}_{3} \mathrm{CCH}_{2} \mathrm{OH}(\mathrm{aq})+\mathrm{Br}^{-}(\mathrm{aq})

[ 6 ]

(i)

State the type of reaction.

[ 1 ]

(ii)

Sketch the mechanism of the reaction, using curly arrows to represent the movement of electron pairs.

[ 3 ]

(iii)

Outline why the reaction takes place by this mechanism.

[ 1 ]

(iv)

Predict, with a reason, a halogenoethane that would react more quickly than bromoethane.

[ 1 ]

0 selected

Exam points

Question 3(c)(iii)

Question 30

Question 4

Question 4(a)

Question 4(b)

Question 39

Question 40

Question 40

Question 4

Question 4(a)(i)

Question 4(a)(ii)

Question 4(c)

Question 4(c)(i)

Question 4(c)(ii)

Question 4(c)(iii)

Question 4(c)(v)